Surgical procedures

ABSTRACT

A method for stereotactic surgical procedure including securing to a predetermined region of anatomy of a patient, at least two stereotaxy frame attachment device. Tomographically scanning the predetermined region to generate scanning data relating to internal and external surfaces of the anatomical region including the frame attachment device and inputting into a data storage device the scanning data.

This invention is concerned with a method and apparatus for carrying outsurgical procedures.

The invention is particularly concerned with the use ofstereolithographic models constructed from tomographic data as apre-operative aid in carrying out the surgical procedure and as apost-operative aid in monitoring a treated patient.

Modelling of anatomical regions using computed tomography data is wellknown for pre-operative planning and rehearsal of procedures and in themanufacture of prosthetic devices.

These modelling techniques employ digitised information from CAD-CAMdesign systems based on data captured and/or reconstructed from avariety of reflection and/or transmission scanning devices. Typically,such scanning device include X-Ray, magnetic resonance imaging (MRI),magnetic resonance angiography (MRA), position emission tomography (PET)as well as ultrasonic imaging.

U.S. Pat. Nos. 4,436,684 and 4,976,737 describe the construction, bymechanical means of anatomical models from computed tomography data.

Of more recent times, stereolithographic modelling has gained prominencedue to its ability to create highly accurate anatomical modelsreplicating both internal and external features of a region underconsideration. In particular, stereolithographically reproduced modelshave been used to permit pre-planning of surgical procedures and, in thecase of plastic and reconstructive cranial surgery in particular, somedegree of rehearsal of surgical procedures is possible.

Stereolithographic and other modelling processes are described in"Paediatric craniofacial surgery: Comparison of milling andstereolithography for 3D model manufacturing Pediatr. Radiol. (1992) 22:458:460; "Computer-Aided Simulation, Analysis and Design in OrthopaedicSurgery", Orthopaedic Clinics of North America--Vol 17, No. 4, October1986; and "Solid models for CT/MR image display: accuracy and utility insurgical planning", Vol 1444 Image Capture, Formatting and Display(1991): 2-8.

Generally speaking, stereolithographic modelling is employed mostly inthe construction of models corresponding to bony tissue although withMRI tomography high resolution imaging of soft tissue has permittedreconstruction of three dimensional images of soft tissue organs fordisplay on monitors. Such displays permit pre-operative planning andmonitoring during surgical procedures such as biopsy sampling,radiotherapy implantation etc. A method of manufacturingstereolithographic models from computed tomography data is described inInternational Patent Application No. PCT/AU94/00536.

In many surgical procedures, particularly intracranial procedures, thereis a need for precise location of soft tissue masses such as malignantlesions, tumours etc relative to other organs such as blood vessels andneural pathways to facilitate pre-operative planning to minimise therisk of accidental trauma to adjacent organs.

One technique for pre-operative planning employs a stereolithographicmodel of a cranial structure in combination with computed tomographydata displayed on a viewing screen. This enables a surgeon to spatiallyvisualise the three dimensional spatial juxtaposition of soft tissuemasses within a cranial cavity and relative to structural featuresthereof.

While this technique is a significant improvement over the mentalanalysis of two dimensionally displayed reconstructions of threedimensional images, there remains an element of "guesswork" in mentalspatial translation of two dimensional images to three dimensionalspace.

Another procedure involves the use of a stereotactic frame associatedwith a patients anatomy as a fixed reference datum in combination withcomputed tomography data to precisely spatially locate selected regionsof soft tissue mass.

Stereotaxis procedures are described in "Guided Microsurgery byComputer-Assisted Three-Dimensional Analysis of Neuroanatomical DataStereotactially Acquired", Stereotact. Funct. Neurosurg. 1990: 54455:482-487; "Three-Dimensional Reconstruction of Neuroradiological Datawithin a Stereotactic Frame of Reference for Surgery of VisibleTargets", Appl. Neuro. 50:77-80 (1987); "The Computer and StereotacticSurgery in Neurological Surgery", Comp. Med. Imag. and Graphics, Vol 12,No. 1 pp75-83 (1988); and, "Integrated Stereotaxis Imaging with CT, MRImaging, and Digital Subtraction Angiography", Radiology 1986;161:821-826.

Stereotaxis surgery is widely used for aspiration and biopsy ofintracranial lesions; implantation of electrodes for sub-corticalstimulation and for the location of seizure foci, thalamotomy formovement; pain and effective disorders; definition of tumour volume forlaser vaporisation; and external beam or interstitial irradiation.

Computed tomography imaging techniques employing X-Ray irradiation areinherently free of geometric distortion and offer good differentiationbetween soft and bony tissues although there are some limitations inresolving soft tissue boundaries where the soft tissues have x-rayattenuation coefficients that differ by less than about 0.5%.

Magnetic Resonance (MR) imaging is free of bone artefact and givesexcellent anatomical detail. Stereotaxy involving MR imaging howeverrequires the use of an attachable frame which does not interact with theimaging process. In consequence such frames suitable for MR imaging areconstructed from quite exotic materials and are very expensive.

In use, stereotaxy frames are fixed to a patient's skull under a generalanaesthetic by means of carbon fibre or aluminium pins in drill holes inthe skull. Locking members retain the pins in place. The frame is thenattached to the pins by locking collars and the patient is thensubjected to an imaging process.

During the imaging process accurate location of the imaged sectionwithin the frame is accomplished by marker sets to establish x, y and zcoordinates within the spatial volume of the frame. Using appropriatecomputer software, the precise spatial location of a tissue massrelative to the frame coordinates permits distance and angularmeasurements and plotting of probe trajectories.

While very effective in surgical procedures, there are a number ofdisadvantages associated with these stereotaxis techniques.

One disadvantage is the need for prolonged anaesthesia whilst a patientundergoes figment of the stereotactic frame, image scanning and then asurgical procedure, although it is possible to disconnect the frameafter imaging and accurately refit it to the mounting pins if a delaybetween imaging and surgery occurs.

Another difficulty is the highly technical nature of the procedure whichis dependent upon specialised computer software and hardware as well asexperienced computer operators. The highly technical nature of thecomputer system can act as a disincentive for many surgeons untrained incomputer technology or otherwise who are intimidated by computertechnology.

A separation of skills between a surgeon and a computer technician insurgical procedures is highly undesirable in stereotaxy as a completeawareness of some anomaly such as mirroring of images may not beapparent to both parties involved in the procedure.

Accordingly, there is a need for a simple yet thoroughly reliablestereotactic technique which is not dependent on computation of data andvisual analysis of two dimensional images for verification of thecomputed data.

It is an aim of the present invention to provide a simple yet reliablestereotactic procedure using a reconstructed three dimensional model tocheck computed data relating to spatial coordinates within the frame orotherwise to utilise the model to set up a stereotactic frame forsurgical procedures.

According to one aspect of the invention there is provided a method forstereotactic surgical procedures comprising the steps of:

securing, to a predetermined region of anatomy of a patient, at leasttwo stereotaxy frame attachment means;

tomographically scanning said predetermined region to generate scanningdata relating to internal and/or external surfaces of said anatomicalregion including said frame attachment means and inputting into a datastorage means said scanning data;

computing said scanning data according to a predetermined algorithm togenerate a three dimensional coordinate data set for the anatomicalregion;

generating from said three dimensional coordinate data set an anatomicalreplica of selected portions of said anatomical region including saidframe attachment means;

attaching to replicated frame attachment means on said anatomicalreplica a stereotaxy frame and aligning instrument support meansassociated with said frame for carrying out a predetermined surgicalprocedure on a selected portion of anatomical region represented by areplication of said selected portions; and

securing said stereotaxy frame to said at least two attachment meansassociated with said predetermined anatomical region of said patient andperforming a surgical procedure on said selected portion of saidanatomical region with pre-aligned instruments supported on saidinstrument support means.

Preferably said frame attachment means are comprised of a materialdetectable by tomography scanning apparatus but otherwise do notinterfere with image data produced thereby.

Suitably, said attachment means comprise non-magnetic material.

If required the attachment means comprise metallic or non metallicelements such as ceramics, carbon fibre, plastics, aluminium and thelike.

The attachment means may comprise pins, plugs, sockets, spigots, clampsor any other suitable means for releasable attachment of a stereotaxyframe to an anatomical region in a spatially reproducible manner.

The attachment means may comprise one or more attachment elements.

Preferably the attachment means comprises at least two attachmentelements.

The attachment means may comprise identical or non identical elements.Tomographic scanning may be carried out with any suitable means forgeneration of scanning data.

Suitably tomographic scanning is unimodal.

If required tomographic scanning may be multi-modal to generate aplurality of separate scanning data sets and/or combined scanning datasets.

The stereotaxy frame may comprise manually operable instrument alignmentmeans. Alternatively, the stereotaxy frame may comprise mechanical orelectro-mechanical instrument alignment means for automated orsemi-automated alignment and/or manipulation of instruments associatedtherewith.

The anatomical replica produced from scanning data may be produced byany suitable means, preferably by stereolithographic modelling.

According to another aspect of the invention there is provided astereotaxy frame comprising:

a base member having at least two spaced apertures for attachment to aregion of anatomical pathology and/or a replicated region of anatomicalpathology; and,

an instrument support means adapted to align a trajectory of saidinstrument according to a selected set of spatial coordinates.

If required the base member may comprise two or more leg members, eachhaving an attachment aperture at a free end thereof.

Suitably the instrument support means is pivotally associated with saidbase member.

Preferably said instrument support means comprises locking means toreleasably secure said support means relative to said base member.

The instrument support means may be pivotally associated with said basemember by a ball and socket joint.

Alternatively the base member may comprise a contoured member having asupport surface complementary to a selected region of anatomicalpathology and a tubular instrument support means fixed relative thereto.

Suitably the contoured member comprises a moulded plastics member havinga tubular instrument guide encapsulated therein.

If required the contoured member may include a plurality of tubularinstrument guides.

In order that the invention may be fully understood and put intopractical effect, reference will now be made to various preferredembodiments illustrated in the accompanying drawings in which:

FIG. 1 shows a side view of a stereotaxy frame in association with astereolithographic model.

FIG. 2 shows a plan view of the arrangement of FIG. 1.

FIG. 3 shows a side view of an alternative embodiment of a stereotaxyframe.

FIG. 4 shows yet another embodiment of a stereotaxy frame.

FIG. 5 shows a further embodiment of a stereotaxy frame.

FIG. 6 shows a stereotaxy frame adapted for bracytherapy procedures.

FIG. 7 shows the fitting of the frame of FIG. 6 to a skull region.

A patient is prepared initially for image scanning for, say, anintra-cranial lesion for preoperative planning of a biopsy and/oraspiration thereof.

Before scanning is effected a stereotaxy frame 1 shown generally in FIG.1 is attached to the patient's skull under a general or localanaesthetic by means of ceramic, carbon fibre or aluminium pins (notshown) located in blind bore holes in the patient's skull. Thestereotaxy frame is then removed from the patient's skull.

The patient is then able to undergo image scanning by say magneticresonance imaging (MRI) or magnetic resonance angiography (MRA) withoutthe stereotaxy frame which may interfere with the imaging process orotherwise give rise to geometric distortions which cannot be correctedfor in computation of the scanning data to recreate two dimensional andthree dimensional images.

The scanning process may utilise a single scanning mode (e.g. MRI) togenerate data or it may be multi-modal (e.g. x-ray CT and MRI) to createseparate or combined images of bony tissue and soft tissue.

After computing the scanning data to obtain reconstructed image datacorresponding to the anatomical region, the image or image data may thenbe edited using known graphics editing techniques to exclude extraneoustissue regions. For example, the outer bony contours of the skull in theregion of interest are retained as is the lesion soft tissue boundaries.Adjacent vascular structures may also be retained and if access to thelesion is necessitated via intracranial bone, this structure may also beretained.

The edited image data is then reconstructed by appropriate computersoftware to generate three dimensional coordinate data for the skullregion of interest including the stereotactic frame mount images andinternal soft and bony structures of interest.

Utilising the three dimensional coordinate data, a stereolithographicreplica 2 of the anatomical pathology is constructed as shown in FIGS. 1and 2. From this replica the surgeon is able to readily determine theshape and size of the lesion 3 and its spatial relationship tointracranial bone plates 4, neurovascular apertures 5 and blood vessels6.

By direct visual inspection of a substantially exact replica of theanatomical region of interest a surgeon is able to carry out a noninvasive "examination" of lesion 3 and determine with precision itsspatial relationship with anatomical features without having to use hisimagination to mentally reconstruct two or three dimensional imagesdisplayed two dimensionally on a computer monitor.

This direct visual examination permits pre-operative planning toaccurately determine the trajectory and depth of penetration of asurgical instrument to minimise trauma along the trajectory path and toavoid damage to blood vessels, nerves and other organs adjacent thelesion site. Where it is necessary to penetrate a bone plate 4 foraccess to lesion 3, the surgeon can directly measure the bone thicknessat the penetration site and also determine whether or not a sinuspassage exists in the region.

Stereotactic frame 1 is then secured to the replica via replicatedattachment sockets 7 and a surgical instrument 8, supported in a guide 9on an articulated arm 10, is then aligned with the lesion 3 along asuitable trajectory. The pivotal joints 11, 12, 13 of arm 10 are clampedto prevent movement or, by means of graduated markings, are able to beaccurately repositioned in a predetermined trajectory after movement.

The surgeon may then practice the surgical procedure on the replicatedmodel whilst being able to visual examine the effects of the procedure.

When the surgeon is ready to carry out the procedure on the patient, thepatient is prepared for theatre, anaesthetised and frame 1 is refittedto the attachment sockets fitted to the patient's skull. By employingsockets or other fittings having differing shapes and/or sizes, thestereotactic frame may only be refitted to the patient's skull in oneposition. Screw threaded frame mounting rods 11a also include markingsor graduations to ensure a precise relocation of the frame on thepatients skull.

The surgical procedure may now be carried out on the patient in anotherwise conventional manner via a small hole drilled in the patient'sskull. The pre-aligned or re-aligned instrument guide 9 then permits thesurgeon to insert instrument 8 into a patient's skull with considerableconfidence as the "guesswork" associated with conventional procedureshas been eliminated as has the risk of computational error incalculating trajectories from scanning data.

FIG. 3 shows an alternative stereotactic frame assembly suitable for themethod according to the invention.

In this embodiment a tripod-like frame attaches directly to framemounting means whereas the embodiment of FIG. 1 comprises a tripod-likeframe attached to a ring frame which in turn attaches to the framemounting means.

FIG. 4 shows yet another form of stereotaxy frame according to theinvention.

In FIG. 4 the frame comprises a base member 20 having tripod legs 21extending therefrom. Adjustable feet 22 are provided at the free end oflegs 21, the feet 22 having mounting apertures 23 therein for attachmentto a region of anatomical pathology 24.

Mounted in a part spherical socket 25 atop legs 21 is a sphericalinstrument support member 26 adapted for pivotal movement within socket25.

Located within support member 26 is a guide tube 27 to guide instrument28 along a predetermined trajectory into the intracranial region 29.

A locking screw 30 is mounted in socket 25 to lock the support member 26in a predetermined position.

FIG. 5 shows a further embodiment of a stereotaxy frame or template 31.

Template 31 may be fabricated by forming a contoured template base 32from scan data to form a stereolithographic analogue (SLA) model of apredetermined region of anatomical pathology of interest.

The template outline may be utilised as a peripheral resection templateor it may be used as a base to support a guide tube 33 for a surgicalinstrument.

After template 31 is formed with reference apertures 34 from a patient'sscan data, the template is secured to the region of interest on the SLAmodel using the replicated reference mounting points. A hole is drilledthrough the template and through the SLA model thereunder to permitinsertion of a surgical instrument (not shown).

When the desired trajectory of the instrument is determined from a probethrough the SLA model, a guide tube 33 is slipped over the top of theinstrument shaft to locate on the surface of the template but otherwisealigned with the longitudinal axis of the instrument. A curable resindough or putty of polyester, acrylic resin or the like 35 is then formedaround the tube 33 to secure the tube 33 in the desired alignmentrelative to template 31.

The template 31 with aligned guide tube 33 is then transferred to thepatient and secured to the patient via screws or the like (not shown)extending through apertures 34.

A drill bit may then be inserted through tube 33 to form an aperture ina bony anatomical pathology such as a skull and a surgical instrumentmay then be guided to a predetermined depth into the intracranial cavityalong a trajectory predetermined by the "practice" alignment on the SLAmodel.

FIG. 6 shows a practical application of another aspect of the inventionto bracytherapy procedures.

A template 40 is modeled using patient scan data to ensure acomplementary mounting surface 41 with a patient's anatomical pathology.

The precise location of a tumour or the like 42 is located relative tothe surface contour of bony mass 43 and a plurality of treatment rods 44are inserted through drilled apertures in the base of template 40 andbony mass 43. Guide tubes (not shown) are then slipped over the freeends of rods 44 to locate within a cavity (not shown) in the body 45 oftemplate 40. A castable resin (not shown) then secures the guide tubeswithin the body 45 in relative angular relationship to guide the rods 44to a predetermined position in the replicated tumour 42.

At the same time, the ends of rods 44 are trimmed or otherwise fittedwith depth stops to ensure a controlled insertion depth.

The template 40, secured to the SLA model is then removed by removingscrews 46 and is secured to the patient's skull at the reference pointsreplicated on the SLA model, again by means of screws 46.

The guide tubes (not shown) secured in the body of template 40 thenpermit the drilling of accurately aligned holes in the patient's skulland subsequent insertion of the rods 44 having radiotherapeutic tipsalong a predetermined trajectory to a predetermined depth to penetratethe tumour 42.

A removable cap 41 is attached to the template body 45 by means ofscrews 48 to retain rods 44 at their initially inserted depth andotherwise to form a hygienic closure. The removable cap 46 permits therods 44 to be changed at periodic intervals with a minimum of trauma tothe patient.

FIG. 7 shows schematically the fixture of template 40 to the body skullstructure 43 of a patient.

EXPERIMENTAL DATA

Several techniques of model guided stereotaxic surgery were developed.Two techniques were firstly investigated as a phantom experiment andthen two similar techniques were carried out on patients.

PHANTOM STUDIES

A cadaveric skull with phantom plaster tumours was marked at four points2 mm holes and CT scanned in a water bath. CT scanning was on a G.E.9800, (General Electric Medical Systems, Milwaukee, U.S.A.).

Images were obtained in bone algorithms 1.5 mm thick and in 1.5 mmcontiguous increments. The data was loaded in a Silicon Graphicscomputer work station which runs ANALYZE v 6.2 3D software (BiomedicalImaging Resource, Mayo Foundation, Rochester U.S.A.) 3D reconstructionof the data performed and the anatomy of interest is edited. BRIDGEWORKSsoftware (Solid Concepts California) is used to generate a support file.

SLA model production occurs as a laser selectively polymerises 0.25 mmslices of photo-sensitive monomer upon a platform that is suspended andprogressively lowered by 0.25 mm into a vat of the liquid monomer aseach slice is polymerised. The SLA model is built up by these contourslices fused one on top of each other. SLA model production time isdependent primarily on the number of slices required for fabrication,the mean time being about 16 hours. The model is then cleaned of supportstructure and hardened in an ultra violet oven.

The SLA model produced was fitted with a stereotaxic frame by way of thefour marker holes. Coordinates for ten "tumour" landmarks were taken onthree occasions. The frame was transferred to the cadaveric skull andthe ten co-ordinates identified on three occasions. The distance of thepointer tip from the target was recorded.

A second form of stereotaxic surgery was also investigated using thecadaveric skull and SLA model. A custom made template may also act as aguide for instruments so as to perform intracranial operations. One ormultiple intracranial trajectories may be chosen and incorporated intothe template by means of barrels. Such a template is made to localisetwo marker points and fit the external contour of the SLA model.

A template was made to fit the phantom SLA model to localise one of thephantom tumours with a wire pointer. Two screws were used to fasten thetemplate to the marker holes found on both cadaveric skull and SLAmodel. The template was then transferred to the cadaveric skull thescrews fastened to the marker holes and the pointer was used to localisethe tumour.

PATIENT STUDIES

Two patients requiring resection of skull based tumours where theresection would leave a cranial defect were selected. The biomodellingprocedure was identical to the phantom study except that the patient wastattooed with Indian ink at four points and E-Z MARK CT markers wereapplied at these tattoos.

SLA models were used to prepare custom acrylic custom implants. Themaster implant may be made from wax or an anatomical section of thenormal side may be mirrored or interpolated to fit the defect. The mastimplant is then hand finished and used to mould and cast an acryliccustom implant.

CASE 1

A custom made stereotaxtic template may also be made to fit an operativeplan on an SLA model so that it traces out a resection margin drawn onthe SLA model by the surgeon allowing the insertion of a prefabricatedimplant. A patient with a large tempero-occipital Tumour was selected.The surgeon marked out the resection margin on the SLA model and acrylicwas moulded so that the edges fitted perfectly into the margin andincorporated the two marker points as two holes. The resection wasrehearsed and the SLA model was used to prepare a customised implant aspreviously described.

The surgeon places the template over the scalp and localises the twotattoos and is held firmly whilst a drill is used to make marker holesin the skull cortex. A scalp flap is then reflected. The template wasthen fixed over an exposed menigioma involving the external temporalregion using the two markers and its contour. The resection margin wastraced from the boundary of the template. The tumour was then resected.The implant fitted to within--1.5 mm.

CASE 2

A stereotaxic frame was used to localise a lytic tumour in thesphenoidal region of a patient. The surgeon marked the ideal resectionmargin out on the SLA mode. The model was located in stereotaxic frameaccording to four marker points. Co ordinates were taken form the marginon the SLA model.

The patient was prepared intraoperatively for stereotaxic surgery. Thetattoo marks were identified on the patient's scalp and used toorientate the stereotaxic frame. The patients scalp was reflected andthe frame used to replicate the co ordinates of the SLA model resectionmargin.

The margin varied by--4 mm from that on the SLA model. Due to thisinaccuracy the implant was used as a template for resection and wasmatched to the surface contour of the patients spinode and had itsboundary marked to replicate the margin on the SLA model. Afterresection the implant was accurately inserted. Inaccuracy beyond that inthe phantom study was due to soft tissue movement during the applicationof the stereotaxic frame.

RESULTS

PHANTOM

The accuracy of localisation was--10.25 mm (mean of thirtylocalisation's) with the stereotaxic frame. Inaccuracy was mainly due tothe +0.85 mm size discrepancy of the SLA model. The use of the templatein the phantom study was within <0.75 mm and was far simpler to performfor a single point.

PATIENTS

These finding were also reflected in the operating theatre with the twopatients. The frame transfer technique had an inaccuracy up to 4 mmwhilst the template transfer was <1.5 mm. The template was found to bemore simple to apply than the frame which because of its cumbersome sizeresulted in some soft tissue movement during application which magnifiedthe degree of error to 4 mm.

CONCLUSIONS

The transfer device for model assisted stereotaxic surgery may takeseveral forms, each with its own merits.

1. A simple impression template may be produced. For example, a mouldedplastic could be used to race out a boundary or to guide an intracranialtrajectory. Such a template could be fabricated on the SLA model andthen transferred to the patient and aligned by the matching of contours.This is a simple device may be difficult to localise if the contour isnot sufficiently distinctive. Attachment of marker pins or screws willhowever overcome this problem as these markers are accurately reproducedon the SLA model.

2. The surface markers can be used to precisely locate the transferdevice which may take the form of a frame or template.

A customised template may be located to both contour and at least twomarkers and be used to race a boundary for a craniotomy so that acustomised acrylic plate can be used to close the defect afterresection. A customised template is located to marker points on the SLAmodel. One or multiple intracranial trajectories may then be chosen andincorporated into the template by means of a barrel. This technique hasspecial application in bracytherapy. A customised template may be madeso that the implants may be encased within the template by means of alid with screws. Such a device can be made to allow the patient to moveabout within the ward easily.

The technique may also be carried out using any standard stereotaxicframe as described. The accuracy of the procedures was comparable to thepresently described stereotaxic techniques. The great source of error inthe phantom study was due to the relative oversize inaccuracy of the SLAmodel by--0.85 mm. This problem may be solved by improving the CTresolution to a spiral generated 512×512 image matrix incorporating ascaling factor to improve SLA model accuracy. Other sources of errorwere apparent in the patient study. These were: soft tissue movement andthe cumbersome nature of the standard stereotaxic frame. Soft tissuemovement can be minimised by reflecting the scalp for templateattachment and by using marker pins for the transfer device to lockonto.

Stereotaxy frames according to the invention will be found in practiceto be ergonomically efficient and simple to use. The device areergonomic in the surgical practice to the model guided stereotaxicsurgery. One device has three point attachment and a base diameter of 10cm. (see diagram) A ball in socket instrument guide would allow a widetrajectory angle and adjustable feet for easy surface attachment. Thisdevice is re-useable and would be compatible with marked SLA models.

The patient is examined by the surgeon and the best position of frame isdetermined using standard imaging data or similar SLA models. The frameis then held against the patients head and the scalp where the framesfeet rest is shave and infiltrated with local anaesthetic.

The frame is held firmly against the patients head and is used to guidethe insertion of three marker points into the bone cortex. The frame isthen removed and the patient undergoes CT/Mr scanning.

An SLA model is created and the frame is attached to the marker pointsincorporated in the model. The trajectory of the procedure is determinedand the frame is locked. The locked frame is then transferred to thepatients hepo and attached to the marker points. This the acts firstlyas a drill guide to breach the patients cranium and then as a guide forthe procedure. The frame can be used easily for multiple trajectorytransfer from model to patient, for multiple biopsies or for marking apre planned resection margin.

These new techniques of model assisted stereotaxic surgery haveadvantages over presently used method for the following reasons:

(a) Being a simple transfer system the skills needed by the surgeon aremechanical and similar to those required for standard operations solittle training is necessary. No computer or computations are necessaryto implement these techniques in the operating theatre as the technologyis hidden in the creation of the exact SLA model.

(b) The frames are simple, cheap and resuable and the only system costfor the hospital. The models are made on a case by case basis.

(c) The patient can be scanned without the frame several days before theprocedure.

(d) Conventional "slice" images of CT/MR data are complex and requiresubjective reconstruction to attain three dimensional understanding. Theaccuracy of such reconstruction is dependent upon the experience andspatial aptitude of the observer. Realistic accurate SLA models providea readily recognisable solid replica of a person's anatomy that requiresno mental reconstruction.

(e) SLA models optimise pre-operative surgical planning and rehearsalbecause a solid anatomical replica may be used realistically andinteractively to simulate surgery e.g. a sterotaxic trajectory. Thisallows the surgeon to plan modification to standard techniques and viewthe planned result.

(f) SLA models provide patients with a clearer understanding of theirpathology and the aims and limitations of surgery pre-operatively andimprove informed consent.

(g) SLA models require no specialised equipment or knowledge forinterpretation and use, are rugged, and may easily be transported andsterilised for intra-operative use.

The study has been carried out at low cost with relatively simplematerials. Advances in SLA biomodelling technology will produce modelswith near perfect accuracy, coloured soft tissue, blood vessels andbone, at low cost. These advances will allow superior results from modelguided sterotaxic surgery in future.

While the invention has been described with reference to an intracranialsurgical procedure, it should be understood that the method may beadapted to a wide variety of surgical procedures such as jointreplacements and delicate procedures such as prostatectomies involvingless invasive procedures than hitherto.

In particular it is believed that the method according to the inventionis particularly suited to robotic surgery techniques wherein the roboticcontrols can "learn" the procedure by practising on an identical replicaof an anatomical region of a patient.

For example a robotic instrument could effect a "craniotomy" of preciseand predetermined dimensions on a stereolithographic replica and then"learn" a surgical procedure such as implantation of electrodes forsub-cortical stimulation. The portion removed from the replica of theskull could then be used to pre-prepare a cranioplasty of slightlylarger peripheral dimensions.

In a subsequent procedure on a patient, the robotic instrument couldcarry out the craniotomy and implantation procedures and the surgeoncould then implant the pre-prepared cranioplasty with minimal manualtrimming to complete the surgical procedure and the minimum of operativedelay.

In addition to or as an alternative to mechanical instruments, thestereotaxy frame may also support alignable radiation emission devicesfor external radiotherapy procedures or for interstitial procedures. Forexample, interstitial hyperthermia may be conducted with a high degreeof accuracy using a precisely located thermal detection device.Similarly this invention permits precise location of radiation emissiondevices for interstitial radiotherapy.

It will be readily apparent to a skilled addressee that manymodifications and variations may be made to the various aspects of theinvention without departing from the spirit and scope thereof.

I claim:
 1. A method for stereotactic surgical procedures comprising thesteps of:securing, to a predetermined region of anatomy of a patient, atleast two stereotaxy frame attachment means; tomographically scanningsaid predetermined region to generate scanning data relating to internaland/or external surfaces of said anatomical region including said frameattachment means and inputting into a data storage means said scanningdata; computing said scanning data according to a predeterminedalgorithm to generate a three dimensional coordinate data set for theanatomical region; generating from said three dimensional coordinatedata set an anatomical replica of selected portions of said anatomicalregion including said frame attachment means; attaching to replicatedframe attachment means on said anatomical replica a stereotaxy frame andaligning instrument support means associated with said frame forcarrying out a predetermined surgical procedure on a selected portion ofanatomical region represented by a replication of said selected portion;and securing said stereotaxy frame to said at least two attachment meansassociated with said predetermined anatomical region of said patient andperforming a surgical procedure on said selected portion of saidanatomical region with pre-aligned instruments supported on saidinstrument support means.
 2. A method as claimed in claim 1 wherein saidframe attachment means are comprised of a material detectable bytomography scanning apparatus but otherwise do not interfere with imagedata produced therebysaid attachment means comprise nonmagneticmaterial.
 3. A method as claimed in claim 2 wherein the attachment meanscomprise metallic or non metallic elements such as ceramics, carbonfibre, plastics, or aluminium.
 4. A method as claimed in claim 3 whereinthe attachment means comprises pins, plugs, sockets, spigots, or clampsfor releasable attachment of said stereotaxy frame to an anatomicalregion in a spatially reproducible manner.
 5. A method as claimed inclaim 4 wherein the attachment means comprises at least two attachmentelements.
 6. A method as claimed in claim 5 wherein the attachment meanscomprises identical or non identical elements.
 7. A method as in claim 1wherein tomographic scanning is carried out with any suitable means forgeneration of scanning data.
 8. A method as in claim 7 whereintomographic scanning is unimodal.
 9. A method as in claim 7 whereintomographic scanning is multi-modal to generate a plurality of separatescanning data sets and/or combined scanning data sets.
 10. A method asin claim 1 wherein the anatomical replica produced from scanning data isproduced by stereolithographic modelling.
 11. A stereotaxy frame for themethod of claim 1, said stereotaxy frame comprising:a base member havingat least two spaced apertures for attachment to a region of anatomicalpathology and/or a replicated region of anatomical pathology; and, aninstrument support means adapted to align a trajectory of saidinstrument according to a selected set of spatial coordinates.
 12. Astereotaxy frame according to claim 11 wherein the base member comprisestwo or more leg members, each having an attachment aperture at a freeend thereof.
 13. A stereotaxy frame according to claim 12 wherein theinstrument support means is pivotally associated with said base member.14. A stereotaxy frame according to claim 13 wherein said instrumentsupport means comprises locking means to releasably secure said supportmeans relative to said base member.
 15. A stereotaxy frame according toclaim 12 wherein is the instrument support means is pivotally associatedwith said base member by a ball and socket joint.
 16. A stereotaxy frameaccording to claim 11 wherein the base member comprises a contouredmember having a support surface complementary to a selected region ofanatomical pathology and a tubular instrument support means fixedrelative thereto.
 17. A stereotaxy frame according to claim 16 whereinthe contoured member comprises a moulded plastics member having atubular instrument guide encapsulated therein.
 18. A stereotaxy frameaccording to claim 17 wherein the contoured member includes a pluralityof tubular instrument guides.